INF1 - E. MECHANISMS OF BACTERIAL RESISTANCE-COVERED

Question 1

what are the 4 ways bacteria achieve resistance

Answer 1

1. degrading antibiotic itself (beta-lactamases)
2. modifying cell permeability properties so antibiotic can’t enter cell
3. modifying cellular target so antibiotic can’t bind
4. efflux pumps

Question 2

why do bacteria develop resistance to antibiotics

Answer 2

• bacteria produce antibiotics to kill off competitors and they must develop resistance mechanisms to these antibiotics molecules (their own)
  SURVIVAL AND SELF-PRESERVATION

Question 3

features of antibiotic resistance

Answer 3

• arises quickly
• spreads fast
• spreads to other bacteria
  (bacteria collect resistance genes)

Question 4

what is intrinsic antibiotic resistance

Answer 4

• occurs at bacterial species level (ie whole species have resistance) and at the kingdom or domain level

1. due to lack of target (ie no cell wall)
2. due to lack of permeability

Question 5

what is acquired antibiotic resistance

Answer 5

• occurs in previously sensitive cells following genetic mutations
• following transfer of genetic information between microbial cells

Question 6

what is a biofilm

Answer 6

• intrinsic resistance to antimicrobials
• micro-environments where bacteria grow as a population
• enables colonisation of a tissue or material allowing the concentration of molecules needed for cell functions (enzymes)
• extracellular matrix of DNA, proteins, polysaccharides etc
• bacteria are harder to eradicate when in a biofilm

Question 7

why do biofilms form

Answer 7

• as a result of quorum sensing: form of chemical communication between bacteria
• allows them to respond to cell density and gene regulation supporting colonisation

Question 8

stages of biofilm formation

Answer 8

1. planktonic bacteria adhere to rough surfaces, adsorption
2. irreversible attachment if humidity, temp, pH, nutrients correct
3. growth and division - biofilm forms (signal molecules ‘talk’ - quorum sensing)
4. mature microcolony forms and there is an extracellular polymeric substance (slime)
5. dispersion: biofilm releases bacteria and cycle starts again or
6. chemoattraction recruits more bacteria to the area to form a multi species consortia (>1 species of bacteria in biofilm)

Question 9

what is transformation

Answer 9

• uptake of naked DNA from environment by a bacterial cell
• not all bacterial strains are transformable (competent)
• competence = cells’ ability to take up DNA genetically determined trait
• competence requires:
  membrane-associated DNA-binding protein
  cell wall autolysis
  various nucleases
• competent strains: Bacillus sp, streptococcus sp, Haemophillus sp, Staphylococcus sp, Pseudomonas sp, Neisseria sp.

Question 10

what are the 4 stages of transformation

Answer 10

1. ds DNA carrying resistance gene approaches competent bacterial cell. Binds to DNA binding protein receptor
2. nuclease degrades one DNA strand whilst other enters the cell
3. DNA inside cell binds competence specific ssDNA-binding proteins
4. RecA protein binds ssDNA and incorporates it in bacterial genome. Cell is resistant

The cell then undergoes cell division and can either retain resistance gene or lose it and cells are sensitive again

Question 11

what is transduction

Answer 11

• DNA transferred from donor cell to recipient cell via a replicating virus (bacteriophage)

Question 12

what is generalised transduction

Answer 12

• any DNA fragment from host genome can be transferred

Question 13

what is specialised transduction

Answer 13

• DNA from a specific region of host chromosome is integrated into virus genome ie- genes encoding for toxins

Question 14

what are the stages of transduction

Answer 14

1. phage injects its DNA
2. phage enzymes degrade host DNA
3. cell synthesises new phages that incorporate phage DNA and mistakenly some host DNA
4. cell lysis releases daughter and transfusing phages
5. transfusing phage injects donor DNA
6. donor DNA is incorporated into recipient’s chromosome by recombination

Question 15

what is conjugation

Answer 15

• transfer of plasmid or chromosomal DNA involving cell-to-cell contact
• equivalent of sexual reproduction
• donor cell must contain a conjugative plasmid (circular pieces of DNA) or chromosomal DNA

Question 16

where do conjugative plasmids (F-plasmid in E.coli) carry the genes

Answer 16

• in the tra-region which regulate the conjugative process
• (eg - genes for pills formation - attaches a bacterial cell to other cells)

Question 17

what does transfer of chromosomal DNA require

Answer 17

1 cell to be a high frequency of recombination cell (Hfr) ie - a bacterium with a conjugative plasmid (for example, the F-factor) integrated into its chromosomal DNA

the conjugative plasmid encodes for necessary machinery to allow conjugation to occur

Question 18

what are the stages of plasmid DNA conjugation (F+ F-)

Answer 18

1. donor (F+) cell attaches to a recipient cell (F-) via the pilus
2. pilus retracts and pulls other cell close to it. F-plasmid is cut at the origin of transfer (oriT)
3. 1 strand of the F plasmid DNA transfers to recipient cell. F- cell starts to synthesise a complementary strand
4. donor cell also synthesises a complementary strand. When DNA synthesis is complete the cell separate (now really both F+ cells)

Question 19

what are the stages of chromosomal DNA conjugation (Hfr F-)

Answer 19

1. F plasmid integrates into chromosome by recombination (this is the Hfr cell)
2. cells join via a conjugation pilus
3. portion of F plasmid partially moves into recipient cell trailing a strand of donor’s DNA
4. conjugation ends with pieces of F plasmid and donor DNA in recipient cell. Cells synthesise complementary DNA strands
5. donor DNA and recipient DNA recombine, making a recombinant F- cell (but does include some material from F+ cell)

DNA transfer begins at omit but movement of cells breaks DNA strand before transfer complete so recipient cell only receives portion of plasmid, remains F- BUT
Has an altered genome - can’t act as a donor